1. Field of the Invention
The present invention relates to liquid crystal materials and eutectic mixtures thereof for infrared and microwave applications. In particular, the invention is directed to a new class of diphenyl-diacetylene liquid crystal compounds and eutectic mixtures.
2. Description of the Related Art
Diphenyl-diacetylene liquid crystals are useful electro-optic media for modulating infrared radiation and for high speed light shutters. These liquid crystal materials possess not only high birefringence but also low rotational viscosity.
The symmetry and polarity of diphenyl-diacetylene liquid crystals are important to the overall properties of the liquid crystal. Properties, such as melting point (T.sub.mp), birefringence (.DELTA.n), viscosity, threshold voltage (V.sub.th), dielectric anisotropy (.DELTA..epsilon.) and heat fusion enthalpy (.DELTA.H), are influenced by the symmetry and polarity of the liquid crystal. These properties are important to the behavior of the liquid crystal in their applications as infrared spatial light modulators and polymer dispersed liquid crystal shutters. A high birefringence improves the light modulation efficiency; low viscosity shortens the response times; and low threshold voyage simplifies the driving electronics in these applications. Moreover, low threshold voltage is particularly attractive for polymer dispersed liquid crystal devices where the applied voltage is partially shielded by the polymer matrix so that the voltage drop across the liquid crystal droplets is far less than the applied voltage. Eutectic mixtures of diphenyldiacetylene liquid crystals are essential to infrared and microwave applications.
Both polar and nonpolar symmetrical diphenyl-diacetylene liquid crystals have been reported in articles by B. Grant, Mol. Cryst. Liq. Cryst., 48, 175 (1978); S. T. Wu et al., J. Appl. Phys., 65, 4372 (1989); and S. T. Wu et al., J. Appl. Phys., 70, 3013 (1991). It is disclosed that the symmetrical diphenyl-diacetylene liquid crystals exhibit high melting temperatures (nonpolar T.sub.mp&gt; 80.degree. C.), narrow nematic temperature range (.about.25 degrees), small dielectric anisotropy (.DELTA..epsilon.0.8) and large heat fusion enthalpy .DELTA.H relative to the ideal hosts for eutectic mixtures. High melting temperature is a result of long conjugation and small dielectric anisotropy is a result of high degree of symmetry of the liquid crystal molecules. Although increasing the alkyl chain length tends to reduce the melting point, disadvantageously, the increase in chain length will increase the viscosity and decrease the dielectric anisotropy.
Polar asymmetrical diphenyl-diacetylene liquid crystals are reported by B. Grant et al., Mol. Cryst. Liq. Cryst., 51, 209 (1979). These liquid crystals have a cyano group attach to a phenyl group on one side and an alkoxy group attached to another phenyl group on the other side of the diacetylene triple-triple bonds. These compounds show a large dielectric anisotropy, but the melting temperatures of these cyano alkoxy diphenyl-diacetylene homologs are exceedingly high (greater than 150.degree. C.) and their nematic range is very narrow (only 5 degrees) relative to the ideal host for eutectic mixtures.
Fluorinated diphenyl-diacetylene liquid crystals are disclosed in German patent No. DE 40 05 882 A1. The German patent does not disclose experimental results or the properties of these fluorinated compounds. The melting temperatures are expected to be high and the nematic ranges thereof are expected to be narrow. Also the dielectric anisotropy (.DELTA..epsilon.) of laterally substituted fluoro-compounds is negative. The alignment for negative dielectric anisotropy, .DELTA..epsilon.liquid crystals is different from the liquid crystal compounds with positive dielectric anisotropy (.DELTA..epsilon.). For infrared and microwave applications, the liquid crystal should have a positive dielectric anisotropy (.DELTA..epsilon.). Therefore, eutectic mixtures consisting of these fluorinated homologs alone are not expected to exhibit a wide nematic temperature range or be practical for infrared and microwave applications.
The ideal host for eutectic mixtures should have properties, such as low viscosity (less than about 30 centipoises), low melting temperature (below 40.degree. C., and preferably about room temperature), wide nematic temperature range (ranging from about -40.degree. C. to +80.degree. C.), low heat fusion enthalpy (less than about 5 kcal/mol) and high birefringence (greater than about 0.25). It would be desirable to have host liquid crystals which have these ideal properties for formulating eutectic mixtures for infrared and microwave applications.